1. Field of the Invention
The present invention relates to a semiconductor device having a bipolar transistor which can drive a large current therethrough and a method of manufacturing the same.
2. Description of the Related Art
Use of a vertical bipolar transistor is often general when a large current is required in a bipolar transistor. Meanwhile, compared to a vertical bipolar transistor, a longitudinal bipolar transistor has an advantage in its simple structure, but has a disadvantage in small drivability for current.
In order to obtain a larger current drivability in a longitudinal bipolar transistor, conventionally, there is employed a method in which a large acceleration voltage for ion implantation of impurity ions into the silicon substrate and thermal diffusion are used to form a larger emitter region and a larger collector region spreading from the surface of the silicon substrate to deep positions at the time of their formation on the silicon substrate, increasing each effective junction area of the emitter region and the collector region, which can permit a larger current to flow.
In order to drive a larger current, there is also employed a method in which V-shaped grooves is formed on a base region formed in a silicon substrate and impurity ions are implanted or diffused into the V-shaped grooves to increase each effective junction area of the emitter region and the collector region (see, for example, Japanese Patent Application Laid-open No. 52-53673).
As described above, in the vertical bipolar transistor, availability in forming large effective junction area of each the emitter region and the collector region per unit area on the surface of the silicon substrate permits a larger current compared to the longitudinal one. In the vertical structure, however, needs for formation of an epitaxial layer in order to form a collector region buried in the silicon substrate, and electrode extension in low resistance from the buried collector region to the surface of the silicon substrate require additional process steps, leading to a problem of complex manufacturing process for the bipolar transistor.
On the other hand, in the longitudinal bipolar transistor, the buried collector region and the epitaxial layer formation are not needed since the emitter region and the collector region are, unlike the vertical bipolar transistor, formed on the surface of the silicon substrate. Further, no electrode extension from the inside of the silicon substrate to the surface of the silicon substrate is also needed. Accordingly compared to the vertical bipolar transistor, a longitudinal bipolar transistor has an advantage in simplified manufacturing process.
It should be noted that, in the conventional longitudinal bipolar transistor, in order to increase the effective junction areas, for example, large acceleration voltage is applied to impurity ions to implant them into deeper positions from the surface of the silicon substrate. In this method, however, a plurality of ion implantations is needed to form the emitter region and the collector region having uniform concentration distribution of impurity ions in a depth direction.
Further, in the ion implantation, requirement for slanted ion implantation, in which impurity ions are implanted from the direction approximately seven degrees tiled from the direction perpendicular to the surface of the silicon substrate to suppress channeling, causes a problem of uniformity in the distance (base width) between the emitter region and the collector region facing each other along the base region in the depth direction.
Further, in the formation of the emitter region and the collector region through ion implantation, the impurity ions cannot be implanted deeper than a certain depth from the surface of the silicon substrate due to either selective ion implantation of impurities with photoresist shaped on the silicon substrate or slanted ion implantation of impurities mentioned above, imposing a limit to the enlargement of the effective junction areas.
Further, in the conventional longitudinal structure, for example, use of thermal diffusion to increase the effective junction areas of the emitter region and the collector region enlarges the both size of the emitter region and the collector region not only in the depth direction but also along the surface direction of the silicon substrate, increasing the areas occupied by these regions on the surface of the silicon substrate, which may result in insufficient effective drivability.
Further, formation of V-shaped grooves, for example, in the base region formed on the silicon substrate is also adopted in some conventional longitudinal structures to increase effective junction areas of the emitter region and the collector region. Even in this structure, formation of a parasitic vertical bipolar transistor causes current flow form the emitter region or the collector region to the silicon substrate via the base region, resulting in consumption as ineffective current.